The invention relates generally to laboratory tables and more particularly to honeycomb tabletops for use in supporting precision laboratory equipment.
Vibration isolation tables used for supporting highly sensitive equipment, such as optical or analytical devices, on a substantially vibration-free surface are well known. Such systems generally include a tabletop comprising metallic upper and lower skins bonded to a honeycomb core and a connecting side wall. The upper skin of the tabletop usually contains a plurality of holes (usually drilled and tapped) which are used for securing equipment on the tabletop surface. A honeycomb core with or without additional stiffening or damping components maintains a rigid separation between the skins and therefore the structural integrity of the top. These tabletops are commonly referred to as honeycomb tabletops and in most instances are supported by a vibration isolation system.
U.S. Pat. No. 4,645,171 is believed to represent the closest prior art to the invention disclosed herein. The '171 patent teaches that cups are secured to the underside of the top skin in registration with the tapped holes. The cups are non-load bearing in that a fastener may be secured in the threaded hole, which is in registration with the cup, without any force or torque being applied to the cup. The tabletops (breadboards) disclosed in that patent are generally referred to in the industry as Clean Top.TM. tabletops. When a spill occurs or the tabletop is otherwise contaminated, the tabletop can be cleaned on site simply by washing with solvents and then rinsing, including aspirating the cups. The cups prevent contamination of the inside of the tabletop.
The present invention is directed to an improvement of the tabletop disclosed in the '171 patent. In prior art constructions, the holes in the top skin are usually tapped and in those constructions the number of threads is limited by the thickness of the upper skin. This is generally suitable for larger breadboards where the thickness may typically range from 0.125 to 0.187 inches. In lightweight breadboards, the thickness of the upper skin is typically 0.074 inches. For these breadboards where the threads are coextensive with the thickness of the upper skin, it is often times difficult to properly secure instruments to such a thin upper skin.
It is essential with breadboards that all movement of the instruments secured thereto, as far as possible, be eliminated. Ordinarily, with the larger breadboards having the thicker upper skin, the thickness of the skin and the number of threads in the skin are sufficient to ensure easily that the instruments secured to the breadboard remain immobile. However, with the lightweight breadboards having the thin upper skin, the threaded fasteners which secure the instruments to the breadboard do not always ensure that the devices will remain immobile.
Threaded fasteners in a tapped hole will frequently loosen or fail due to the metal fatigue of the fastener or stripping of the threads in the hole. Most often the failure of the fastener will occur at the threads which are at the end of the tapped hole closest to the application of the load, i.e. the top surface of the skin. When the load is transferred from a threaded fastener to the threads in a tapped hole in the usual way (threaded fastener in tension, tapped hole in compression) the transfer occurs in a nonuniform manner along the thread engagement between the threaded fastener and the tapped hole. The load transfer will be maximum at the first engaged thread and the direction of the load will decrease, but not linearly, to a minimum at the last engaged thread. With the thin upper skin and depending upon the instrument secured thereto, there is sometimes not enough thickness and/or number of threads to properly secure the fastener.
In the present invention, tapped holes in an upper skin of a lightweight breadboard are formed to both thicken the skin in that portion of the tapped hole closest to the applied load and to form a sleeve depending from the lower surface of the upper skin. This latter feature allows for more threads to be formed in the hole to secure an instrument to the top skin. Further, the holes as formed enhance the strength and integrity of the top skin.
Plastic or metal cups are glued into position after the top skin has been fully manufactured. The shape of the extruded hole keeps the adhesive out of the threads.